A directional sound pressure gradient microphone includes a front acoustic terminal for leading a sound wave from a sound source to the front and the back surfaces of a vibration plate and a rear acoustic terminal. An acoustic resistor is mounted in an acoustic path from the rear acoustic terminal to the back surface of the vibration plate for generating a predetermined sound propagation-delay time.
A typical directional microphone of a prior art will be described referring to FIGS. 6a and 6b. FIG. 6a is a front view illustrated from the side of a front acoustic terminal of the directional microphone. FIG. 6b is a sectional view of FIG. 6a taken along the line A—A.
According to FIGS. 6a and 6b, the microphone includes a columned microphone case 10 made of aluminum or the like. The front face of the microphone case includes a predetermined number of a front acoustic terminal 11 which has an opening. The front acoustic terminal 11 generally has a metallic mesh.
The microphone case 10 includes a vibration plate 12 strained and supported by a support ring (diaphragm ring) 12, and a fixed electrode 13 supported by an insulating pedestal 13a. The vibration plate 12 and the fixed electrode 13 are faced and disposed through a spacer 12b with a predetermined gap obtained between the plate 12 and the electrode 13. An electret is generally put on the fixed electrode in a small capacitor microphone.
The insulating pedestal 13a is called “cylinder” and is a disc-formed molded component made of synthetic resin fitted in the microphone case 10. The rear face of the insulating pedestal 13a has a rear acoustic terminal 14. The pedestal has a plurality of acoustic paths 15 leading a sound wave from the rear acoustic terminal 14 to the back face of the vibration plate 12. For example, each of the paths 15 is coaxially arranged and equally spaced apart.
A recess 16 which communicates with each of the acoustic paths 15 and which has a much larger diameter than that of each of the paths 15 is disc-formed at the side of the support of the fixed electrode in the insulating pedestal. The acoustic resistor 17 made of a nylon mesh or the like as well as a damper 18 are housed in the recess 16. The damper 18 is an elastic body such as sponge having air permeability and is used for pressing the acoustic resistor 17 to the bottom of the recess 16.
A circuit substrate 20 having an impedance converter 21, for which a FET (Field Effect Transistor) is used in this example, is disposed at the rear face side of the insulating pedestal 13a. The pedestal as well as the circuit substrate are fixed by caulking an opening end of the microphone case 10.
A contact terminal 23 is disposed in the center portion of the insulating pedestal 13a for electrically connecting the fixed electrode 13 to the FET 21. In this example, a contact spring 24 is mounted between the contact terminal 23 and the fixed electrode 13. The FET 21 is disposed on the circuit substrate 20 through a support cushion 22. The elastic contact of the contact terminal 23 to the gate of the FET 21 obtains high reliability of the connection.
In the capacitor microphone having a structure described above, the acoustic resistor 17 directly affects the directional frequency response, the gain and the signal-to-noise ratio Therefore, it is very important that the variability of the acoustic resistance in the acoustic resistor 17 is designed to be as small as possible.
The variability of the acoustic resistance is mainly generated by sound leakage from the gap between the acoustic resistor 17 and the recess 16 housing the resistor. That is, the variability of the acoustic resistance occurs by the sound which avoids the acoustic resistor 17 and which propagates from the bottom to the side of the recess 16 and which passes to the rear surface of the vibration plate 12.
To solve the problem, the variability of the acoustic resistance is suppressed by sufficiently increasing the inner diameter of the recess 16 and the contact area between the recess and the acoustic resistor 17. However, for example, in a capacitor microphone having a diameter of a little over 10 mm, the recess 16 housing the acoustic resistor 17 becomes small in proportion to the diameter of the microphone and a sufficiently large contact area of the acoustic resistor 17 and the recess cannot be obtained. Therefore, the variability of the acoustic resistance generates the variability in the directional frequency response, the gain and the signal-to-noise ratio.